During combustion processes, nitrogen in the air and/or fuel is thermally oxidized into nitric oxide (NO) or nitrogen dioxide (NO2). Combined, these products of reaction are referred to as mono-nitrogen oxides (NOx). NOx emissions are a significant pervasive pollutant that causes a wide variety of diseases, contributes to ozone and smog formation, causes 20 to 30 percent of acid rain, and is the basis for visibility problems because of the formation of aerosols.
NOx is generally produced by a thermal NOx process or a fuel NOx process. Thermal NOx is produced from the oxidation of nitrogen (N2) at temperatures above 1500° F. and is thus related to high flame temperatures. Fuel NOx results from oxidation of nitrogen organically bound in the fuel.
Thermal NOx is the primary source of NOx formation when natural gas and distillate oils are used for combustion because these fuels contain generally lower nitrogen content or are devoid of nitrogen altogether. Sources of thermal NOx include automobiles, power plants, industrial furnaces, and boilers. Petroleum refining represents about 5% of the total NOx emissions which are often concentrated in small geographic areas known as non-attainment areas wherein unusually high concentrations of NOx and concomitant ozone may accumulate. It is has been and continues to be a matter of environmental and commercial interest to reduce NOx emissions in furnaces used in the petroleum refining industry such a hydrocarbon cracking furnaces, especially in non-attainment areas.
There are two primary technology paths for reducing NOx. A first technology is to utilize selective catalytic reduction (SCR) to convert the exhaust NO to N2 and H2O by reacting with ammonia. A second technology is to utilize low NOx burners to reduce the formation of NOx during combustion. SCR implementations, while very effective, can be cost prohibitive. The subject of the present invention relates to a more cost effective low NOx burner.
Low NOx burners may employ a number of methods to reduce NOx. A first method is staging the combustion process where fuel rich and fuel lean zones are established within the flame. The fuel rich zone is the primary combustion zone and prevents the formation of thermal NOx by providing a low oxygen concentration. Fuel lean zones have generally lower flame temperatures than stoichiometric flames which result in lower thermal NOx and lower fuel NOx.
Since NOx formation is highly dependent upon flame temperature, small reductions in flame temperatures can dramatically lower NOx formation. This is generally accomplished by diluting the fuel-air mixture with inert material such as flue-gases or with steam. Flue gases are just the gaseous combustion products of the flame. Burners that utilize the former technique are classified as FGR (flue-gas-recirculation) burners. Injection of steam is accomplished typically by a) injecting steam into the source combustion air prior to mixing with the fuel, or (b) injecting the steam into the fuel itself, prior to mixing with the combustion air.
Ultra low NOx burners aim to lower the NOx emissions below 30 ppm for industrial furnaces with the ultimate goal of reaching better than 5 ppm. Several references disclose devices and methods for lowering NOx emissions in fuel burners. Pre-mixing of steam with gaseous or atomized liquid fuel or combustion air is known.
For instance, U.S. Pat. No. 3,907,488 to Takahashi et al discloses two sets of heavy oil burners including a fuel rich burner and a fuel lean burner arranged in an alternating pattern so as to operate both burners in a regime that produces lower NOx. The burners utilize vaporized water to create and cool gasified combustion oil. Light oil burners utilize a spray of water surrounding the gasified fuel jet.
A second example is U.S. Pat. No. 5,983,622 to Newburry, et al., where steam is premixed with fuel in a diffusion flame combustor by spraying the liquid fuel stream into a flow of steam to produce a combined fuel/steam flow. Newburry teaches away from separately injecting fuel and steam into diffusion flame combustors to decrease the peak flame temperature to lower the production of NOx. Newburry claims that uneven distributions of oil and steam into a combustor result in locally hot and cold regions, thereby causing a higher production of CO.
U.S. Pat. No. 6,986,658 to Stephens, et al. discloses premixing of the fuel and steam in a burner for steam cracking. The apparatus provides a method for mixing combustion air, flue gas and steam to reduce NOx emissions by reducing the temperature of the mixture.
Other prior art discloses the introduction of steam from a port separate from the fuel port and intermixing the steam and fuel in the combustion chamber. One example is U.S. Pat. No. 7,104,069 to Martling, et al. This reference discloses a method for providing a regulated steam flow to fuel nozzle assemblies for a gas turbine combustor. Steam is injected separately from the fuel in an annular array adjacent the fuel nozzle.
Another example is U.S. Pat. No. 5,285,628 to Korenburg. Korenburg discloses fuel combusted first in a first fuel lean region and then again in a fuel rich region. Steam and air is injected into each combustion region to reduce NOx formation. The staged combustion apparatus is designed to operate a gas turbine.
U.S. Pat. No. 4,533,314 to Herberling shows steam used as a cooling gas, introduced into a combustion chamber so that steam mixes with the combustion gas in approximately equal amounts with the flue gas in an interleaving arrangement whereby the concentration of steam is maximized at the flame front.
U.S. Pat. No. 5,344,307 to Schwartz et al discloses methods and apparatus for burning fuel with low NOx formation including a primary reaction zone, a secondary reaction zone and a steam injection nozzle disposed within the burner housing below the primary and secondary reaction zones.
The injection of steam is seemingly an inexpensive way to reduce NOx formation in many types of burners, including gas turbines. However, steam injection has some drawbacks. For gas burners it is known that the premixing of steam into gaseous fuel lines causes corrosion in the carbon steel lines and excess degradation due to fuel gas condensation so that the fuel gas lines must be cleaned periodically or replaced with stainless steel to prevent corrosion. This can be costly and time consuming.
Furthermore when pre-mixing of steam into gaseous fuel lines, less fuel will flow through the small fuel tips, which require larger openings to use the same volume of fuel and therefore achieve similar heating rates. Prior art burners fail to optimize burner nozzles to lower NOx levels by steam-fuel mixtures. Therefore, the nozzle openings must be modified to allow proper target fuel and steam flow. During modification, the burner performance may be compromised as nozzle opening size, location and direction are parameters engineered for a particular performance by the manufacturer.
The injection of steam into combustion air requires that steam be distributed evenly throughout the combustion region, which poses problems for the large burner assemblies of industrial furnaces. Furthermore, in comparison to the premixed fuel/steam method, a much larger volume of steam is required to achieve comparable NOx emission results.
In practice, there is a need for ultra low NOx burners for industrial furnaces, in particular hydrocarbon cracking furnaces that incorporate steam addition at a location other than in the air/fuel premix region or in the fuel itself.